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Thallium (Tl)

Thallium is a chemical element of the periodic table with chemical symbol Tl and atomic number 81 with an atomic weight of 204.382 u and is classed as post-transition metal and is part of group 13 (boron group). Thallium is solid at room temperature.

Thallium in the periodic table

Atomic number81
Group13 (Boron group)
ClassificationPost-Transition Metal
AppearanceSilvery white
Color Silver
Number of protons81 p+
Number of neutrons123 n0
Number of electrons81 e-
From Wikipedia, the free encyclopediaThallium is a chemical element with symbol Tl and atomic number 81. This soft gray post-transition metal is not found free in nature. When isolated, it resembles tin, but discolors when exposed to air.

Physical properties

Phase at STPSolid
Density11.85 g/cm3
Atomic weight204.382 u

Thermal properties

Melting point577 K
303.85 °C
578.93 °F
Boiling point1746 K
1472.85 °C
2683.13 °F
Heat of vaporization162.09 kJ/mol

Atomic properties

Electronegativity (Pauling Scale)1.62
Electron affinity36.4 kJ/mol
Oxidation states−5, −2, −1, +1, +2, +3
(a mildly basic oxide)
Ionization energies
  1. 589.4 kJ/mol
  2. 1971 kJ/mol
  3. 2878 kJ/mol

Electron configuration for thallium

Electron configuration
Shorthand configuration
[Xe] 4f14 5d10 6s2 6p1
Electron configuration
Full configuration
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p1
Electron configuration chart
Electrons per shell2, 8, 18, 32, 18, 3
Valence electrons 3
Valency electrons 1,3
Bohr model
ThalliumElectron shell for Thallium, created by Injosoft ABTl
Figure: Shell diagram of Thallium (Tl) atom.
Orbital Diagram

The history of Thallium

DiscoveryWilliam Crookes (1861)
First isolationClaude-Auguste Lamy (1862)
Discovery of thallium
Thallium (Greek θαλλός, thallos, meaning "a green shoot or twig") was discovered by William Crookes and Claude Auguste Lamy, working independently, both using flame spectroscopy (Crookes was first to publish his findings, on March 30, 1861). The name comes from thallium's bright green spectral emission lines. As both scientists discovered thallium independently and a large part of the work, especially the isolation of the metallic thallium was done by Lamy, Crookes tried to secure his own priority on the work. Lamy was awarded a medal at the International Exhibition in London 1862: For the discovery of a new and abundant source of thallium and after heavy protest Crookes also received a medal: thallium, for the discovery of the new element.


List of unique identifiers for Thallium in various chemical registry databases
CAS Number7440-28-0
ChemSpider ID4514293
EC number231-138-1
PubChem CID Number5359464